A power converter is a power processing circuit that converts an input voltage waveform into a specified output voltage waveform. In many applications requiring a DC output, switched-mode DC/DC converters are frequently employed to advantage. DC/DC converters generally include an inverter, a transformer, a rectifier on a secondary side of the transformer and an output filter. The inverter generally includes switching devices (inverter switches), such as field effect transistors (FETs), that convert a DC input voltage to an AC voltage. The transformer transforms the AC voltage to another value and the rectifier delivers the desired DC voltage to the output filter. Conventionally, the rectifier includes a plurality of rectifying diodes that conduct the load current only when the rectifying diodes are forward-biased in response to the input waveform to the rectifier. Finally, the output filter eliminates large fluctuations in the output voltage to provide a substantially constant DC voltage at the output of the converter.
A typical power converter embodying the principles as described above is a voltage-fed power converter, such as half-bridge power converter. A conventional half-bridge power converter includes two inverter switches coupled to a control circuit, at least one transformer, a rectifier and a filter. This type of power converter generally operates as follows. The first and second inverter switches conduct current in a complimentary manner to convert an input DC voltage into an AC voltage to be applied across the primary windings of the transformer. The rectifier then rectifies the voltage from the secondary windings of the transformer and the filter smooths and filters the rectified voltage to develop an output voltage for delivery to a load. The control circuit monitors a characteristic (e.g., the output voltage) of the power converter and adjusts the duty cycle of the inverter switches to ultimately control the output voltage. The output voltage may be maintained at a relatively constant level despite relative fluctuations in the input voltage and the load. There are many possible methods of regulating such power converters including, for instance, pulse width modulation (PWM). PWM is one of the more widely used control and switching methods and, as such, will not be herein discussed.
The conversion efficiency of conventional regulated power converters is related to the duty cycle of the inverter switches. To achieve maximum conversion efficiency in the conventional power converter described above, the inverter switches usually should be operated at a full duty cycle. For example, in a half-bridge power converter, each inverter switches would be on for about 50% of the duty cycle (e.g., in a symmetrical mode of operation). However, the constant operation of the inverter switches does not allow the output voltage to be regulated in response to variations, for example, in the load or the input voltage. To regulate the output voltage, the inverter switches in conventional power converters, such as the half-bridge converter described above, should be operated in an asymmetrical mode of operation or a symmetrical mode of operation (with reduced duty cycles) , wherein the duty cycles of the inverter switches are adjusted to regulate the output voltage.
In asymmetrical half-bridge converters, one of the inverter switches has a first duty cycle (D) while the other inverter switch has a second duty cycle (1-D). In this methodology, if the first duty cycle (D) is less than 50%, the second duty cycle (1-D) would be greater than 50%. In symmetrical half-bridge converters, both inverter switches are operated with a duty cycle (D). However, the duty cycle (D) is necessarily below the full duty cycle.
One problem with operating a power converter asymmetrically is the substantial power loss across at least one of the inverter switches and at least one of the rectifying diodes. Regulating the output voltage of an asymmetrical power converter results in unnecessary power losses because at least one inverter switch is exposed to a substantially larger voltage than the other inverter switch. In addition, at least one of the diodes used in the rectifier is also exposed to similar high voltage stresses. Specifically, when the converter is working at its highest input voltage, one of the inverter switches and one of the diodes in the asymmetrical half-bridge power converter may be subjected to approximately twice the voltage required during low voltage operation. Thus, one of the inverter switches, as well as one of the diodes, must be substantially larger than their respective counterparts to withstand this high voltage stress. The larger components necessary to withstand the increased voltage stress, however, increase the cost of manufacturing the power converter. Further, the larger components may tend to incur a greater power loss thereacross.
The inverter switches in a symmetrical converter should be operated below full duty cycle. Reducing the duty cycle of the inverter switches invariably results in a corresponding time period wherein the power converter is not processing power ("dead time") Whenever a power converter operates with any significant amount of dead time, the power converter is not operating at maximum efficiency.
Accordingly, what is needed in the art is a voltage-fed power converter capable of operating at a substantially full duty cycle (symmetrical operation), while maintaining the ability to regulate the output voltage to the desired level.